arm64/mm: avoid fixmap race condition when create pud mapping

arm64/mm: avoid fixmap race condition when create pud mapping

The 'fixmap' is a global resource and is used recursively by
create pud mapping(), leading to a potential race condition in the
presence of a concurrent call to alloc_init_pud():

kernel_init thread                          virtio-mem workqueue thread
==================                          ===========================

  alloc_init_pud(...)                       alloc_init_pud(...)
  pudp = pud_set_fixmap_offset(...)         pudp = pud_set_fixmap_offset(...)
  READ_ONCE(*pudp)
  pud_clear_fixmap(...)
                                            READ_ONCE(*pudp) // CRASH!

As kernel may sleep during creating pud mapping, introduce a mutex lock to
serialise use of the fixmap entries by alloc_init_pud(). However, there is
no need for locking in early boot stage and it doesn't work well with
KASLR enabled when early boot. So, enable lock when system_state doesn't
equal to "SYSTEM_BOOTING".

Signed-off-by: Jianyong Wu <[email protected]>
Reviewed-by: Catalin Marinas <[email protected]>
Fixes: f4710445458c ("arm64: mm: use fixmap when creating page tables")
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Will Deacon <[email protected]>